Method of producing nanophase WC powder by vapor phase reaction

ABSTRACT

Nanophase WC powder is produced by preparing a precursor including tungsten; producing gas by vaporizing or sublimating the precursor; carbonizing the gas in the atmosphere without oxygen while maintaining pressure below atmospheric pressure; and condensing the carbonized gas

TECHNICAL FIELD

[0001] The present invention relates to a method of producing nanophasepowder, which is used as cemented carbide requiring high strength andwear-resistance, or materials for high-speed tool steel, heat-resistancesteel etc., or more particularly, to a method of producing WC powder ofgrade of several ten nanometers from a precursor containing tungsten bymeans of vapor phase reaction.

BACKGROUND OF THE INVENTION

[0002] Generally WC powder is produced in most cases by solid statereaction in which W powder and solid state carbon powder are mixed andcarbonized at high temperature. However, the prior art method users astart material in solid state and so requires mixing and millingprocess. Also the process is complicated due to many steps in theprocess of oxidized W, and time consuming due to the interaction betweensolid state materials in the growth of WC powder.

[0003] Meanwhile, liquid state methods have been used to produce finepowder of WC/Co by spray-drying solution including W and Co by usingwater-soluble metal base. However, these methods require manycomplicated process. Also further, with these types of methods, thereare limitations in producing ultra-fine powder of 0.1 μm or less due tothe growth of WC powder in the carbonizing heat treatment.

SUMMARY OF THE INVENTION

[0004] The present invention purports to provide a method of producingWC cemented carbide powder of approximately 10 nm or below by a simplerprocess by using vapor phase reaction.

[0005] In order to accomplish this objective, with respect to the methodof producing WC powder from a precursor containing tungsten, the presentinvention comprises the steps of preparing a precursor containingtungsten; producing gas by vaporizing or sublimating said precursor;carbonizing said gas in the atmosphere without oxygen while maintainingpressure below atmospheric pressure; and condensing said carbonized gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a flowchart for producing nanophase WC powder accordingto the present invention.

[0007]FIG. 2 is a structural diagram, which illustrates the apparatusfor producing nonophase powder, which is used in the production methodof the present invention.

[0008]FIG. 3 is a photograph, which shows the structure of nanophasepowder produced according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] The present invention is described in further detail as below.The present invention comprises producing nanophase powder by directlyvaporizing or sublimating a precursor containing tungsten, and then bycarbonizing and heat-treating said gas at the pressure below atmosphericpressure.

[0010] Any precursors as long as they contain tungsten suffice as saidprecursor, and liquid-phase precursors, such as tungsten ethoxidesolution (V solution) or tungsten chloride (WCl₆) solution, orsolid-phase precursors, such as tungsten hexacarbonyl (W(CO)₆), may beused. Or another element such as Co can be added to the solution ifnecessary.

[0011] The present invention comprises vaporizing or sublimating saidprecursor into gas, and then carbonizing said gas in the atmospherewithout oxygen while maintaining pressure below atmospheric pressure.

[0012]FIG. 1 is a flowchart for producing nanophase WC powder accordingto the present invention, and FIG. 2 is a structural diagram, whichexemplifies the apparatus for producing nanophase powder according tothe present invention for vaporizing said precursor, followed byseparation and condensation of the tungsten component.

[0013] As illustrated in FIG. 2, the apparatus 100 for producingnanophase powder by vapor phase reaction comprises a vaporizer 10, whichvaporizes the precursor 1 fed from a storage vessel by means a pump (notillustrated); a reactor 20, which separates the tungsten component byheating said vaporized precursor; and a condenser 30 connected to saidreactor 20.

[0014] The vaporizer 10 is connected with a carrier-gas feed pipe 2, anda mixed-gas feed pipe 3, which discharges the mixed gas of vaporizedprecursors and carrier gas. The vaporizer 10 feeds the mixed gas to thereactor 20.

[0015] The reactor 20 is connected with a reactor adjustor 21, which canadjust the temperature of the reactor. Further, a reactor valve 15 isinstalled between the vaporizer 10 and the reactor 20 for adjusting theflow rate of carrier gas.

[0016] Upon opening said reactor valve 15 for producing WC powder, themixed gas of the vaporized precursor and carrier gas is fed to thereactor 20, and then the gas is carbonized at a pressure belowatmospheric pressure. The carbonized gas is supplied to the condenser 30for condensation and collection, and the remainder gas is discharged tothe discharge pipe 32.

[0017] The main characteristics of the present invention are as follows:By means of carbonization of precursor gas, which is in gas phase at amolecular level, at a pressure below atmospheric pressure, thecarbonization reaction rate is fast, and by the same token, the size ofthe end-product powder, condensed after the completion of thecarbonization reaction, is approximately 10 nm or less in nanophase.

[0018] The pressure for carbonization reaction is set preferably at1.3×10⁻⁵˜1 atm. According to the present invention, it is possible toobtain fine WC powder under carbonization reaction pressure of roompressure of 1 tm. However, it the reaction pressure is maintained at lowpressure of less than 1 atm, the reaction rate could be furtherincreased. Yet, to maintain ultra-low pressure of less than 1.3×10⁻⁵atm, it is problematic in terms of cost, and therefore it is necessaryto maintain the above pressure range.

[0019] The feed pipes (2,3) can be made of metal, such as stainlesssteel or copper, or ceramics or Teflon, such as alumina, mullite orsilicon carbide. It is preferable to use a material which can withstanda temperature of 50˜300° C. in the range of vaporization temperature ofthe precursor 1. Further, the vaporizer 10 can be made of a stainlesssteel tube, alumina tube, quartz tube, or pyrex tube, with one endblocked off, which can withstand the vaporization temperature of theprecursor.

[0020] Carrier gas can be selected, at least one, from CO, CO₂, CH₄,C₂H₄, He, Ar, N₂, or H₂, or the mixture thereof, which can form inertatmosphere, and the flow rate of gas of approximately 10˜2,000 cc/min isappropriate.

[0021] Meanwhile, in case of using liquid-phase precursors, the flowrate of 0.05˜2 cc/min is appropriate.

[0022] The reactor 20, in the shape of a horizontal tube, can be made ofa stainless steel tube, quartz tube, mullite tube, alumina tube, etc.The reactor 20 is equipped with a heater. The reaction gas(carbonization gas) including C component is introduced into the reactorto react with the gasified precursor.

[0023] When CO, CO₂, CH₄, C₂H₄ is used as carrier gas introduction ofcarbonization gas is not necessary since the carrier gas transportingthe precursor gas in the vaporizer 10 and pipe 3 can be used as acarbonization gas in the reactor 20. Since the temperature in thereactor 20 is maintained which is sufficient for carbonization reaction,the carrier gas can be used in the carbonization.

[0024] The atmosphere in the reactor 20 is maintained without oxygen bymeans of carrier gas. In the present invention, carbonization is carriedout preferably at the temperature of 500-1,500° C., or more preferablyat 1,000˜1,200° C. If it is 500° C. or less, the carbonization reactiondoes not occur actively, and in the interest of product yield and costreduction, the upper limit should be preferably kept at 1,500° C.

[0025] The gas carbonized in the reactor 20 are fed into the condenser30, in which the gas sinks naturally and condenses, or absorbs to thesurface of a cooler installed within the condenser and condensesthereafter. The cooler is filled with cooling medium of temperaturebelow zero, such as cooling water, liquid nitrogen or liquid helium, andby using such cooler, by way of the so-called thermophoresis effect, theabsorption is carried out much faster than that of condensation bynatural sinking. If the cooler is rotated, it further results insuperior condensation efficiency.

[0026] Other elements such as Co, Mo, V, Ni, Cr or Fe can be added tothe precursor.

[0027] Below, the present invention is described in further detailthrough an example. The example is for illustration purposes only and isnot intended to limit the present invention to any specific form. It maybe readily known to those skilled in the art that the present inventionis not restricted to the example. It is intended that the scope of thepresent invention be defined by the claims appended hereto and theirequivalents.

EXAMPLE

[0028] Tungsten hexacarbonyl, which is a non-corrosive solid-phaseprecursor, with a vaporization temperature of 170° C., was prepared andfed through the apparatus of FIG. 2. In feed, it was vaporized and thenfed into the reactor of an alumina tube of an outer diameter ofapproximately 40 mm and an inner diameter of approximately 30 mm. CO gaswas used as carrier gas.

[0029] The vaporized precursor is carbonized and condensed in thereactor under various temperature and pressure. Table 1 shows theresult. TABLE 1 Carbonization Size of powder Temperature (° C.) particlePressure(atm) Example 600 4 1.3 × 10⁻² 1000 5 Comparative 600 53 1Example

[0030] As can be seen in table 1, the powder of example, which iscarbonized at low pressure, shows fine structure compared with thepowder of comparative example, which is carbonized at atmosphericpressure.

[0031]FIG. 3 shows the picture of WC powder collected aftercarbonization. As can be seen in the picture, the size of WC powderproduced by the method of the present invention is about 4 nm smallerthan 10 nm.

INDUSTRIAL APPLICABILITY

[0032] As described in the specification, the present invention providesadvantages in that the present invention is a simple process since itdirectly carbonizes tungsten in gas phase by vaporizing or sublimatingthe tungsten precursor.

[0033] Further, the present invention provides WC powder of grade ofseveral ten nm by carbonization and condensation of molecular-level gasin vacuum by means of vapor reaction, and the nanophase powder producedthereby has high-strength and excellent wear-resistance, which can besuitably used as cemented carbide such as for carbide tools, or asmaterials for wear-resistance components or metal molds.

What is claimed is:
 1. A method of producing nanophase WC powder byvapor phase reaction, which comprises the steps of preparing a precursorincluding tungsten; producing gas by vaporizing or sublimating saidprecursor; carbonizing said gas in the atmosphere without oxygen whilemaintaining pressure below atmospheric pressure; and condensing saidcarbonized gas.
 2. The method of producing nanophase WC powder by vaporphase reaction according to claim 1, wherein said precursor, at leastone, is selected from the group consisting of tungsten hexthoxide,tungsten chloride, and tungsten hexacarbonyl.
 3. The method of producingnanophase WC powder by vapor phase reaction according to claim 2,wherein said atmosphere without oxygen comprises at least one of CO,CO₂, CH₄, C₂H₄, He, Ar, N₂, or H₂, or the mixture thereof.
 4. The methodof producing nanophase WC powder by vapor phase reaction according tothe claim 3, wherein said step of carbonization is carried out at thetemperature of 500˜1,500° C.
 5. The method of producing nanophase WCpowder by vapor phase reaction according to the claim 4, wherein saidcarbonized gas is condensed under the pressure below atmosphericpressure.
 6. The method of producing nanophase WC powder by vapor phasereaction according to the claim 5, wherein said carbonized gas iscondensed by absorbing the same onto the surface of a cooler at thetemperature below zero.
 7. The method of producing nanophase WC powderby vapor phase reaction according to claim 2, wherein said atmospherewithout oxygen comprises at least one of CO, CO₂, CH₄, C₂H₄, He, Ar, N₂,or H₂, or the mixture thereof.
 8. The method of producing nanophase WCpowder by vapor phase reaction according to the claim 1, wherein saidstep of carbonization is carried out at the temperature of 500˜1,500° C.9. The method of producing nanophase WC powder by vapor phase reactionaccording to the claim 1, wherein said carbonized gas is condensed underthe pressure below atmospheric pressure.
 10. The method of producingnanophase WC powder by vapor phase reaction according to the claim 1,wherein said carbonized gas is condensed by absorbing the same onto thesurface of a cooler at the temperature below zero.